Explanation:
such an equation is a simple linear function :
y = ax + b
b is in this case the flat fee, and a (the slope) drives the incline of the function for various numbers of students x.
our job is now to find the correct a and b values.
we do this by using the informative we have and create 2 equations for a and b, and then we solve this system.
132.5 = a×25 + b
145 = a×30 + b
now, we can use a little trick and simply subtract the whole first equation from the second equation.
145 = a×30 + b
- 132.5 = a×25 + b
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12.5 = a×5 + 0
5a = 12.5
a = 2.5
with this we go into one of the original equations, e.g. the second one :
145 = 2.5×30 + b = 75 + b
70 = b
so, it full equation is
y = 2.5x + 70
the costs for 85 students are
y = 2.5×85 + 70 = 212.5 + 70 = $282.50